How can i attach the knife to the BCD?

[SkimFisher]

Since you want a knife that you can attach to your BCD so it will be readily accessible and you don't have to take it on and off between dives, which is a good idea..then..um..get a BCD knife rather than one designed to place on your arm or leg and then make changes to it. You aren't going to be able to improve on the manufacturers design.

If you are concerned about being stuck with an unneeded piece of gear that you bought but now regret, there's a classified section on this board and there's Ebay.

Sell it and get yourself a knife better designed for your purpose.

What he said. Oceanic Spinner is a good choice - among others.